ANDRE’S GROOVED CORE MYTH
April 19, 2009
Q: There is a perception out there that Andre only recommends grooved and perforated core, and never uses a surface flow medium as it infringes a patent. But I thought you were over that and if it made sense you did it.
A: W@#$%????? (Initial reaction edited for improper language), let’s start again:
That perception is wrong.
There is an obvious problem with verbal communication resulting in translation (which leads to myths: we are still looking for the lost civilization of Atlantis).
I take it that there are no written notes of what we did during each of the Advanced CMT training sessions, or this perception about “Andre only recommends grooved and perforated core, and never uses a surface flow medium as it infringes a patent” would not be out there, it is a patently wrong perception!
I always use, explain and demonstrate the same three ‘flow assist methods’ during the courses: surface medium, interlaminar medium and core grooves. I also always explain in detail the pro’s and con’s of each.
As for applications: I recommend the use of a surface medium in projects were weight is critical and flow characteristics of the stack are suitable.
Patents are irrelevant to my technical approach; selection of the infusion method and the flow medium is based on the laminate configuration, the performance requirements, the weight criteria, the mold quality and, last but not least: the degree of sophistication at the point of use: skill level of the users, control of the shop environment, suitability of the vacuum system and general understanding of the process. This is especially critical in carbon laminates, and has been emphasized in all my lectures and demonstrated in the lab.
That said, it is up to the end user to decide which method they want to use, and they need to do their own research on potential patent infringement, and be careful about how they want to describe the process: for instance, SCRIMP™ is a trademarked and patented system that requires, as far as I know, a license and associated fees.
Case in point about the challenges of carbon: a lab session where we applied a substantial carbon laminate to both sides of a thick core. The selected surface medium created significant lag problems due its high flow rate combined with the selected carbon fabric, which had very poor z-axis resin penetration characteristics.
We tried every combination possible to avoid/minimize the use of core grooves, but the lag factor between top and bottom laminates remained unacceptable.
The only practical and timely solution was to get the resin under the top laminate and flow it through customized minimal size grooves, resulting in a minor weight penalty.
The course text that I used in the all my courses is on my www.GRPguru.com website under the “VIP Update” Tab. (Past tense indeed: Under the ‘Seminars and Courses’ Tab you’ll find that I no longer have a course program posted.)
It is a bit dated, but still very valid and might make for good reading. It also lists the SCRIMP™ and other patents. You will notice that SCRIMP™ patent # 4,902,215 (US) was issued on 2/20/1990.
US utility patents filed before June 1995 are valid for 17 years from the date of acceptance….
Get out that TI 82 calculator and do the math: 2009 minus 1990 equals?
However #1:
I do favor grooved core for most cored laminate applications. I suggest that you make your engineers and crews build a boat hull with incorporated stringers, and do it once with surface medium and once with grooved core. Have them record: time, materials and supplies cost, and make them weigh the finished product. They’ll see why grooved core is not so bad after all, for most applications.
However #2:
I do favor using interlaminar flow for most single skin laminates: using a surface medium leads to significant compaction of the laminate stack under vacuum, thus creating the sometimes desired high fiber volume fractions (FVF), but at the cost of the section thickness. Section loss is typically in the order of 30%, which affects the laminate stiffness significantly. Rebuilding the section with more reinforcements usually results in a heavier laminate than designed, with an unbalanced stiffness/strength design safety factor.
OK, get the TI 82 back out and calculate the % of stiffness loss after a 30% section decrease , using the rule that stiffness increases/decreases as the cube of the thickness. If your primary goal is to have the highest possible FVF for a single skin laminate, this is not an issue, as long as the engineering is taking both factors into account.
For most ‘conversion’ laminates however, the open mold laminate was tested and proven, has a specific thickness and weight. Why mess with perfection in that case? The switch to closed molding is as easy as this: By switching the old style open mold fabrics to newly developed closed mold fabrics, and in addition inserting a reinforcement with a lower SG and/or FVF such as a non-woven, a braided fabric, a CFM, or other specialty open weave fabric as close to the neutral axis as possible, you can offset the loss of section without increasing the overall laminate weight, at the same time maintaining or improving the original laminate specs.
Common sense and dollars, not myths!
Sorry, a bit longer answer than my initial reaction, but the teacher in me got the upper hand over the impulsive guy I sometimes turn out to be. I think that is a good thing.
Keep the questions coming.
